1. Field of the Invention
The present invention relates to an LED array head and a circuit board, and LED array chips that form the LED array head, whereby the LED array head is used as a light source for writing an electrostatic latent image on a photoconductive drum in an electrophotographic printer.
2. Description of the Related Art
With a conventional electrophotographic printer, a charging roller charges the surface of a photoconductive drum and an exposing unit such as an LED head writes an electrostatic latent image on the charged surface of the photoconductive drum. The LED array head emits light through a focusing rod lens array to illuminate the charged surface in accordance with print data. The electrostatic latent image is developed with toner into a toner image, which is subsequently transferred to a print medium. The toner image on the print medium is then fixed by a fixing unit.
FIG. 8 illustrates a general construction of an electrophotographic printer.
Referring to FIG. 8, there is provided a photoconductive drum 1 surrounded by a charging roller 2, an LED array head 3, a focusing rod lens 4, a developing roller 5, and a transfer roller 7. Arrows indicate directions of rotation of the structural elements. The developing roller 5 applies toner to the electrostatic latent image on the photoconductive drum to develop the electrostatic latent image into a toner image. The transfer roller 7 transfers the toner image from the photoconductive drum 1 to a print medium 100. A supply roller 6 rotates in contact with the developing roller 5 to supply toner to the developing roller 5. A fixing device 101 fixes the toner image transferred to the print medium 100.
The conventional LED array head 3 will be described in detail.
FIG. 9A is a perspective view illustrating the general construction of the LED array head 3 when the driver chips 10 are arranged on one side of the row of the LED array chips 9.
The circuit board 8 has conductive pattern i.e., wiring pattern formed thereon. Each of the LED array chips 9 has a plurality of light-emitting diodes fabricated therein. Driver chips 10 are connected to the corresponding LED array chips 9 through wires 11, and drives the LED array chips 9. The driver chips 10 are also connected to the circuit board 8 through wires 12.
The driver chips 10 may be arranged on both sides of the row of the LED array chips 9 as shown in FIG. 9B.
FIGS. 10A and 10B illustrate the construction of the conventional LED array head 3, FIG. 10A being a top view and FIG. 10B being a cross-sectional view taken along lines 10Bxe2x80x9410B.
FIG. 11 is a perspective view showing the structure of the conventional LED array chip 9.
Referring to FIGS. 10A, 10B, and 11, the LED array chip 9 has light-emitting elements 13 and individual electrodes 14a formed thereon. The individual electrode pads 14b are formed on the LED array chip 9 by using the same material as the individual electrodes 14a. The individual electrode pads 14b and drive electrode pads 15 are provided for wire bonding. The LED array chip 9 has common electrode 23 formed on the underside thereof.
Each of driver chips 10 is formed with drive electrode pads 15 thereon. Signal inputting and outputting pads 16 are formed on the driver chips 10 for wire bonding. The LED array chips 9 and driver chips 10 are bonded on the circuit board 8 by an adhesive 28 that contains conductive particles therein. The circuit board is formed with conductive pattern 17 thereon.
The circuit board 8 takes the form of a glass epoxy board with copper conductive pattern 17 formed thereon. The driver chip 10 is formed on a silicon substrate, and the LED array chip 9 is formed on a compound semiconductor in which gallium arsenide phosphide is grown on a gallium arsenide substrate by epitaxy. The common electrode 23 formed on the bottom side of the LED array chip 9 is formed of gold or gold alloy. Wires 11 and 12 are gold.
The circuit board 8 carries as many LED array chips as there are driver chips 10. The LED array chips 9 are connected to the driver chips 10 by using as many wires 11 as there are light-emitting elements. For example, if an image is to be printed with a resolution of 600 dpi on A4 size paper, 26 LED array chips 9 and 26 driver chips 10 are required to be mounted on the circuit board 8. Each LED array chip 9 has 192 light-emitting elements aligned at intervals of 42.3 xcexcm and 192 wires 11 are used to connect individual electrode pads on the LED array chip 9 to corresponding drive electrode pads on the driver chips 10.
The common electrodes 23 on the LED array chips 9 are electrically connected to the conductive pattern 17 on the circuit board 8 by means of an adhesive 28 containing conductive particles. The adhesive 28 is a thermosetting epoxy resin type adhesive. Once the adhesive 28 sets, the conductive particles are sandwiched between the common electrode 23 and the conductive pattern 17, thereby establishing electrical continuity therebetween.
The driver chips 10 are fixedly mounted on the circuit board 8 using an insulating epoxy resin type adhesive. The signal inputting and outputting pads 16 of the driver chips 10 are connected to corresponding conductive pattern (wiring pattern), not shown, on the circuit board 8. The driver chips 10 receive electrical signal and drive the corresponding LED array chips 9 in accordance with the electrical signal, thereby selectively causing the light-emitting elements 13 to emit light.
The assembly operation of the LED array head 3 will be described.
First, the adhesive 28 is applied to the conductive pattern 17 formed on the circuit board and then the LED array chips 9 are mounted on the conductive pattern 17 using the die-bonding apparatus. Then, the insulating epoxy resin type adhesive is applied to predetermined areas of the circuit board 8 where the driver chips 10 are to be mounted, and then the driver chips 10 are mounted thereon.
The light-emitting elements are arranged at intervals of 42.3 xcexcm for the resolution of 600 dpi. For good print results, adjacent LED array chips 9 must be positioned such that the distance L2 between the endmost light-emitting elements of adjacent LED chips is exactly the same as the distance L1 between adjacent light-emitting elements within the adjacent LED array chips. The distance L1 is closely controlled to be 42.3 xcexcm. Using the die-bonding apparatus, the LED array chips 9 are positioned with respect to the alignment patterns such that the distance L2 is accurately 42.3 xcexcm.
After having the LED array chips 9 and driver chips 10 bonded thereon, the circuit board 8 is placed in an oven. The circuit board is heated at 150xc2x0 C. in the oven, so that the adhesive 28 sets to fix the LED array chips 9 and driver chips 10 in position. Then, the circuit board 8 is taken out of the oven and is cooled. The conductive particles contained in the adhesive 28 has a diameter of several microns. Heating the adhesive 28 causes the conductive particles to be coupled to one another, thereby making the electrical connection between the LED array chips 9 and the circuit board 8.
After the die-bonding of the chips, individual electrode pads of the LED array chips 9 are wire-bonded to the corresponding drive electrode pads of the driver chips 10. The signal inputting and outputting pads 16 of the driver chips 10 are wire-bonded to the conductive pattern 17 formed on the circuit board 8. This completes the assembly of the LED array head 3.
The thus assembled conventional LED array head 3 presents the following problem.
When the adhesive 28 is cooled down after the heating process, the dimension K of the LED array chip 9 in the direction in which the light-emitting elements are aligned becomes shorter. This is due to the fact that the adhesive 28 shrinks and there is a difference in thermal expansion between the LED array chip 9 and circuit board 8.
FIG. 12 illustrates the shrinkage of the LED array chip 9 when the adhesive sets.
The dimension K of the LED array chip 9 is K=Ka and the light-emitting elements are aligned at intervals of L1=L1a (=Ka/192).
Using the adhesive 28, a plurality of LED array chips 9 are mounted on the circuit board 8 such that the distance L2 between endmost light-emitting elements of adjacent LED chips is L2a. When the circuit board 8 is heated, the circuit board 8 expands more than the LED array chip 9 and the LED array chip 9 is bonded on the thermally expanded board 8. When the circuit board 8 is cooled down, the circuit board 8 shrinks to the original size. As the circuit board 8 shrinks, the LED array chip 9 is subjected to a shrinking stress such that the dimension K shrinks by xcex94K to K=Kb=Kaxe2x88x92xcex94K. This causes the distance L2 to become longer by xcex94K to L2=L2b=L2a+xcex94K. In other words, each of the adjacent LED array chips shrinks such that the opposing longitudinal ends of the LED array chip 9 shrinks by xcex94K/2 toward the center of the LED array chip. Therefore, the distance L2 becomes longer by 2(xcex94K/2)=xcex94K. The distance L1 becomes shorter by xcex94K/191 so that the dimension L1=L1axe2x88x92(xcex94K/191).
The LED array chip 9 having 192 light-emitting elements designed for 600 dpi has the dimension Ka of 8.1 mm before the adhesive sets. Therefore, the shrinkage xcex94K is about 4 xcexcm, which represents about 9% of the desired distance L=42.3 xcexcm between adjacent light-emitting elements. After the adhesive has set, the distance L2 increases to about 46.3 xcexcm. If the distance L2 becomes too longer than a desired value L, white thin lines appear in the print results.
White lines that appear in the print result will be described with reference to FIGS. 13 and 14.
FIG. 13 illustrates odd-numbered light-emitting elements being energized and even-numbered light-emitting elements not being energized when the LED array chip has expanded such that light-emitting elements are aligned at intervals of L2=L+xcex94K.
FIG. 14 illustrates a printed pattern of lines and spaces when a printing is performed using the LED array head energized as shown in FIG. 13.
Referring to FIG. 14, a width LN of each line is constant and a width S of each space is also constant. A space S+xcex94K, which corresponds to the distance between endmost light-emitting elements of adjacent chips, is wider than the space S. It is to be noted that the white area is larger in the region corresponding to the distance L2 than in the other regions corresponding to the distance L1. The larger white areas result in white lines in the printed results.
FIG. 15 illustrates,the relationship between the probability of occurrence of white lines and the ratio xcex94K/L, where L is a desired distance L2 between the adjacent light-emitting elements of adjacent chips and xcex94K is a deviation or increase from the desired distance L.
A printing is performed by using a plurality of LED array heads which have the same distance L1 (=42.3 xcexcm) but different distance L2. Then, a plurality of persons inspect the printed patterns. The probability of occurrence of white lines is defined by the ratio of the number of persons who found white lines to the total number of persons. The results shown in FIG. 15 reveal that white lines are noticed when the expansion (inter-chip space) xcex94K/L exceeds 0.08 or 8% depending on the characteristics of the printer under test.
As mentioned above, with the conventional LED array head, white lines can appear due to increased distances L2 between endmost light-emitting elements of adjacent LED array chips.
The adhesive not applied evenly or the LED array chips not sufficiently pressed against the circuit board 8 causes increases in the distance between the common electrode 23 of the LED array chip 9 and the conductive pattern formed on the circuit board 8. This in turn causes poor electrical contact between the common electrode 23 and the conductive pattern 17, sometimes the electrical contact being completely lost. Moreover, poor electrical contact may occur some time after the LED array head has passed the testing.
The present invention was made in view of the above-described problems with the conventional art.
An object of the invention is to reduce the shrinkage of the LED array chips after the adhesive has set, thereby improving the print quality.
Another object of the invention is to ensure the electrical continuity between the common electrodes of the LED array chips and the conductive pattern formed on the circuit board, thereby improving reliability of the LED array head.
An LED array head comprises a circuit board and a plurality of LED array chips bonded thereon. Each of the plurality of LED array chips has a plurality of light-emitting elements aligned and exposed on a surface of the LED array chip. The plurality of LED array chips are aligned on the circuit board in a direction in which the plurality of light-emitting elements are aligned such that the light-emitting elements lie on a single straight line. The LED array chips are bonded by an epoxy resin type soft adhesive to the circuit board.
Each of the plurality of LED array chips has a common electrode that is connected to all of the light-emitting elements. The common electrode is electrically connected through the soft adhesive to the conductive pattern formed on the circuit. The conductive pattern may include bumps and dips. The soft adhesive contains conductive particles therein.
Alternatively, the common electrode may have bumps and dips formed thereon.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.